This invention relates to a method of quantitatively determining the grain size d of substances having an ultra-sound attenuation coefficient .alpha.= .alpha..sub.A+ .alpha.S, wherein the absorption coefficient .alpha..sub.A depends linearly upon the frequency f of the ultrasound (.alpha..sub.A = a.sub.1.sup.. f) and the coefficient of scatter .alpha..sub.S is H.d.sup.3. f.sup.4= a.sub.4.sup.. f.sup.4 (for H= const (density, speed of sound and anisotropy)), by measuring the pressure amplitude A.sub.S (x) of the scattered sound as a function of the transit time of the sound through the specimen, and averaging over limited regions of structure by relative continuous or discontinuous movement between the source of the sound and the specimen.
Whereas the scatter of electromagnetic waves (X-rays, visible light, radar) has already found many uses in practice no advantage has as yet been taken of the possibility of utilizing the scatter of sound waves.
Ultrasonic pulses in steel are partly absorbed by the material and partly they excite the crystals (grains) of the structure to re-radiate detectable quantities of the sound (= scatter).
In the conventional technique of assessing the structure of a material by ultrasound the attenuation in planoparallel specimens due to absorption and scatter is measured and the result is evaluated. Scatter measurements call for a different experimental arrangement:
Ultrasonic bursts (frequencies between 5 and 25 MHz) are applied through a liquid entry path (e.g. through water) at an angle of incidence exceeding the angle of total reflection of the longitudinal wave, so that only the transversal wave is propagated through the material. The grains of the structure scatter the ultrasound in every direction but some of it is returned to the source (usually a piezoelectric material, such as quartz, lithium sulphate and so forth). The returning high frequency signals are amplified, converted to digital form in a high speed analog-to-digital converter (conversion rate about 100 MHz) and stored in a computer. Relative movement between the measuring head and the material (circular, elliptical or linear) yields different signals at different times from different parts of the structure. This averaging process is needed in order to eliminate signals caused by interference (from crystallites in particularly favorable or unfavorable locations. Rectification of the mean values provides a scatter amplitude distribution A.sub.S (x) as a function of the path length x of the sound (calculated from velocity of the sound and its transit time).
For homogeneous workpieces EQU A.sub.S (x)= const..sup.. .sqroot..alpha..sub.S.sup.. e.sup..sup.-.sup..alpha.x
Where .alpha..sub.S = coefficient of scatter
.alpha.= COEFFICIENT OF ATTENUATION ATTENUATION= .alpha..sub.A (absorption)+ .alpha..sub.S.
The constant contains all the parameters of external effects. The representation of log.sub.e A.sub.S (x) on a plotter indicates homogeneity by the linearity of EQU log.sub.e A.sub.S (x) .about. -.alpha..sub.x.
Deviations from the straight line mean that there are inhomogeneities in the material.
The drawback of this procedure is that a calibrated specimen having a known type of structure is needed and this latter structure must be determined by other means. Hence the examination of different materials always requires the availability of corresponding calibrated specimens. (J. Koppelmann, Materialprufung 9 (1967), p.401, and J. Koppelmann, Materialprufung 14 (1972), p.156. Also B. Fay, Acoustica, vol. 28 (1973), p. 354).